1. Field of the Invention
The present invention relates to the field of Global Positioning System (GPS) receivers receiving transmission GPS signals from satellites and processing the GPS signals for obtaining information, and storing the information. The present invention particularly relates to a GPS receiver tracking system tracking a high dynamic vehicle in accordance with GPS data received from a translator placed on the high dynamic vehicle. The system is preferably implemented entirely using software.
2. Description of the Related Art
The Global Positioning System (GPS) is a satellite-based navigation system that continuously transmits timing, frequency and satellite position information to potential users. The GPS consists of a full constellation of four satellites in half geo-synchronous orbits. The GPS satellites continuously emit coded GPS signals The GPS signals can be received from at least four GPS satellites at any point on or near the Earth.
A GPS signal contains timing information that allows a user to determine the time elapsed for the GPS signal to transverse the distance between the GPS satellite and a receiver receiving the signal. By knowing the time the GPS signal left the GPS satellite, the time the GPS signal arrived at the user, and the speed of the GPS signal, the receiver can determine the distance from itself to the GPS satellite. The measured range is referred to as “pseudorange” because there is generally a time difference or offset between timing clocks on the satellites and the GPS receiver clock. Thus, for three dimensional position determinations, at least four satellite signals are needed to solve for four unknowns, i.e., the time offset and the three dimensional positions of the satellites. By knowing the orbital position of four GPS satellites (ephemeris data), and the distance from itself to each of four GPS satellites, the receiver can successfully triangulate its own position. Receiver position calculations performed using greater than four satellites generally have an improved accuracy.
The GPS signal emitted by the satellites contains an L-band carrier component (L1) transmitted at a frequency of 1575.42 MHz. The L1 carrier component is modulated by a coarse acquisition (C/A) pseudorandom (PRN) code component unique to the satellite and a data component. The PRN code provides timing information for determining when the GPS signal was broadcast and identifying which satellite emitted the signal. The data component provides information defining the satellite ephemeris, satellite clock corrections and other GPS information. The carrier component allows a receiver to more easily acquire the GPS signal.
It is known in the art to use the Global Positioning System (GPS) for testing and evaluating purposes, such as for testing and evaluating the performance of moving vehicles. However, accurate development of test data for high dynamic vehicles has proved problematic, as well as for test data obtained from multipath GPS signals and GPS signals reflected from the ocean.
GPS receivers have been developed for testing of high dynamic vehicles. However, the receivers are limited for use with very high dynamic vehicles, such as missiles having a short time of flight and small dimensions. GPS receivers are generally too large to be fitted on a small missile, and the time needed for obtaining a first set of data may exceed the entire flight time.
Typically, use of GPS receivers for platforms such as small missiles has been replaced by the use of translators. A translator is a device capable of downlinking the complete GPS spectrum to ground recording equipment for post flight tracking and trajectory reconstruction. The translators have applications for high dynamic guidance, range safety and post-test trajectory analysis. However, translators currently available tend to be costly and highly proprietary, limiting the possibility of modifying the translator for special applications. Another drawback to available translators is the need for accessing a large bandwidth that is not always available at test ranges.
In one suggested solution, recorded data obtained from the translator and recorded data obtained by a ground receiver approximately simultaneously with the data obtained by the translator are processed by filtering and smoothing algorithms to produce a smooth trajectory. However, during the boost phase of the flight, the signal-to-noise ratio (SNR) is very low.
Accordingly, there exists a need for a method and a system for processing recorded GPS data obtained form a translator mounted on a mobile dynamic vehicle and recorded data obtained approximately simultaneously with the data obtained by the translator by a ground receiver, in which the tracking results are accurate for all phases of movement of the vehicle, where the vehicle is a high dynamic vehicle.